The bipolar membrane is a composite membrane in which a cation-exchange membrane and an anion-exchange membrane are stuck together, and has a function for splitting water into protons and hydroxide ions.
By utilizing this special function, the bipolar membrane is incorporated in an electric dialyzing apparatus together with the cation-exchange membrane and/or the anion-exchange membrane. Upon conducting the electrodialysis, an acid and an alkali can be produced from a neutral salt. Therefore, a variety of applications have been proposed (non-patent document 1).
The above bipolar membrane requires a high degree of adhesion particularly between the cation-exchange membrane and the anion-exchange membrane, and it has been desired that the membranes are effectively prevented from being swollen even after the membranes are used, for example, for the electrodialysis for extended periods of time and that the electrodialysis is stably conducted without permitting the membranes to be peeled off. Therefore, a variety of manufacturing methods have been proposed.
For example, patent document 1 proposes a method of sticking a cation-exchange membrane and an anion-exchange membrane together using a mixture of polyethyleneimine and epichlorohydrin and adhering them together by curing.
Patent document 2 proposes a method of adhering a cation-exchange membrane and an anion-exchange membrane together using an ion exchange adhesive.
Patent document 3 proposes a method of applying a fine powdery ion-exchange resin (a paste-like mixture of an anion- or cation-exchange resin and a thermoplastic material) onto a cation-exchange membrane or an anion-exchange membrane, and press-adhering them together.
Patent document 4 proposes a method of production by applying a paste-like material comprising a vinylpyridine and an epoxy compound onto the surface of a cation-exchange membrane followed by the irradiation with radiant rays.
Patent document 5 proposes a method of applying a sulfonic acid-type high molecular electrolyte and allylamines onto the surface of an anion-exchange membrane, and effecting the crosslinking by the irradiation with ionizing radiant rays.
However, though the adhesion of these membranes was improved, the bipolar voltage was so high that they were not industrially useful. The bipolar voltage (V) is expressed by the sum of a water splitting voltage (V′) required for splitting water in the interface between the cation-exchange resin layer and the anion-exchange resin layer constituting the bipolar membrane and potential differences (VC, VA) occurring in relation to the electric resistances of the layers. The theoretical voltage of the water splitting voltage V′ is about 0.83 V, and attempts have been made to lower the bipolar voltage by bringing the water splitting voltage V′ that constitutes the bipolar voltage as close to the theoretical voltage as possible. The attempts are to place a chemical material having a catalytic function chiefly for accelerating the splitting of water in the interface between the two layers where the splitting of water takes place, and have contributed to lowering the bipolar voltage.
For example, non-patent document 2 and patent document 6 disclose making heavy metal ions present in the interface of the two layers.
Non-patent document 3 discloses forming a layer having a tertiary pyridine as an intermediate layer between the two layers.
Further, patent document 7 and non-patent document 3 disclose making an inorganic ion-exchanger present in the interface between the two layers